It is common practice in semiconductor manufacturing facilities and chemical plants to supply a plurality of gaseous raw materials at prescribed flow rates for chemical reaction in a reaction furnace to produce high purity target gases or reactants. The pressure of the gaseous raw materials is detected and regulated, and the gas flow rate is controlled automatically at a predetermined value. The gas pressure needs to be detected with great accuracy in order to regulate the gas flow rate with high precision.
A pressure transducer is normally employed to measure gas pressure, but where a highly accurate measurement is needed, an oscillatory pressure sensor is employed.
Amongst the oscillatory pressure sensors known in the art is a silicon-semiconductor oscillatory pressure sensor disclosed by Japanese patent publication Hei 4-68574.
Said silicon-semiconductor oscillatory pressure sensor is made as follows: a silicon substrate is used as a gas pressure detecting diaphragm, an H-shaped oscillator is embedded in the silicon substrate by means of the semiconductor planar technique, and the oscillator-embedded silicon substrate is then encased in a semiconductor-made capsule, the interior of which capsule is evacuated. The semiconductor planar technique allows the oscillatory pressure sensor to be made extremely small. When used for measuring pressure in the micro-size field, a pressure sensor is normally referred to as a “micro-pressure sensor”.
The pressure measuring principle of the silicon-semiconductor oscillatory pressure sensor is as follows: if the silicon substrate diaphragm is subjected to pressure whilst a forced oscillation at a prescribed frequency is applied to the oscillator embedded in the diaphragm, then the oscillator incorporated is affected such that its resonant frequency is altered. This change in resonant frequency is put to use in the present invention. Because a specific relationship exists between the quantum of the change in the resonant frequency and the applied pressure, the pressure can be determined by measuring the change in the resonant frequency.
In order to apply a forced oscillation to said H-shaped oscillator, a current-conducting lead-in wire is formed in the silicon substrate and capsule by means of the planar technique, and a permanent magnet is installed outside of the oscillator to apply a magnetic field to the oscillator. An alternating current is passed through said lead-in wire to generate an alternating electromagnetic force as the current passes through said magnetic field, thereby subjecting the oscillator to a forced oscillation by the action of the alternating electromagnetic force.